Slime Moulds: the most remarkable organisms.

by Sarah Lloyd

Leocarpus fragilis

'Slime moulds' is a not term that elicits excitement in most
people, nor does it conjure up images of great beauty. But slime
moulds must be among the most remarkable of organisms! At one stage
of their life they are single cell amoeba, whose definition is found
in a Dictionary of Zoology, then they combine with others of their
kind to form either a plasmodium - or pseudoplasmodium - defined in
the Dictionary of Plant Sciences.

Fuligo septica

My fascination with slime moulds has been growing gradually since
first reading about them in books about fungi. (They were once
placed in the same kingdom as fungi but are now in their own
kingdom: the Protoctista.) Their sudden appearance is particularly
intriguing. On one occasion I went outside to find three fruiting
bodies in various colours of Fuligo septica on logs or
stumps about 20-50 metres apart. This left me wondering about the
stimulus for their sudden appearance.

In an attempt to find out more about slime moulds I purchased The
Social Amoebae: the biology of cellular slime moulds, a small book
written by John Tyler Bonner. Bonner (aka the 'sultan of slime')
is professor emeritus of ecology and evolutionary biology at
Princeton University who has worked and written about his 'beloved
slime molds' for six decades. He writes in the preface that one
reason for the book was to clarify in his own mind the complex
nature of the subject of his life's work. But rather than
clarifying it for me, the information, initially at least, was
bewildering. Then, after reading Virgil Hubregtse's account of a
talk given by Paul George (Fungimap #28), I realised the reason for
my confusion: there are three types of slime moulds! There are the
Dictyostelids, the cellular slime moulds so eloquently written about
by Bonner (featured on the ABC Science Show 29th Aug 2009), the very
obscure Protostelids, and the acellular slime moulds, also known as
plasmodial slime moulds or myxomycetes. The cellular slime moulds
are mostly microscopic; the acellular slime moulds are the ones we
see.

Myxomycetes: Plasmodial or Acellular Slime Moulds

One of the most frequently encountered acellular slime mould is the
aforementioned Fuligo septica. Its common names of either 'dog
vomit' or 'scrambled egg' slime mould evocatively describe its
size and consistency. It appears on rotting logs, stumps or live
vegetation during summer, first as a moist brightly coloured
(usually yellow) blob, then, as the spores develop, it fades and
gradually hardens. It is likely, given that many acellular slime
moulds have a cosmopolitan distribution, that it was the one that
featured in 9th century Chinese writings called 'Kwei hi' which
translates to ‘demon droppings’. In an area of Mexico the
plasmodium is fried and eaten by some of the indigenous people who
call it 'caca de luna' i.e. 'moon's excrement'.

Ceratiomyxa fruticulosa

Other slime moulds have quite different forms. From a distance
Ceratiomyxa fruticulosa is no more than a white splash on
rotting stumps and logs, but closer inspection reveals an intricate
architecture of miniature icicles. When it first
appears Stemonitis axifera resembles a collection of small
shiny beads. These gradually elongate and change colour before
transforming into a brown fluffy spore-bearing mass. The fruiting
body of Lycogala epidendrum, whose common name is 'wolf's
milk', are 3 - 15mm orbs of pink, red or orange which gradually
change to pinkish grey.

Arcyria obvelata

What really got me hooked was finding a colony of exquisite 4mm
fruiting bodies resembling tiny deep purple mushrooms that were
scattered along the trunk of a dogwood (Pomaderris apetala) that had
been lying on swampy ground for years, possibly decades. After
checking a few websites the distinctive appearance of the slime
mould made it easy to identify as Arcyria denudata.

I replaced the slime mould in a shady spot and planned to make
regular visits to record its progress. As luck would have it, there
was another Arcyria species about a metre away that I could
also monitor.

I have learnt quite bit about slime moulds since that encounter
with the purple Arcyria. For instance, slime moulds are
apparently very sensitive to disturbance (they don't like rough
handling, but they don't seem to mind loud exclamations of delight
on being discovered!) and although a few of the A. denudata
fruiting bodies on the sodden dogwood matured, I lost track of most
of them and presume they did not cope well with being moved. Another
mistake I made was photographing the very early stages which can be
similar in different species. For example, many fruiting bodies
first appear as bright yellow plasmodia, or a collection of small
beads or stalked cylinders of jelly. It is only when these mature
that their identifying features become obvious. However in many
instances, as with fungi, microscopic examination of spores and
other structures is needed for identification.

Surprisingly, there have been only about 1000 species of slime
moulds recorded worldwide (in comparison, there are believed to be
approximately one million fungi). They reach their peak of abundance
in temperate forests and can be found on living and dead trees,
rotting logs and other coarse woody debris, leaf litter, herbivore
dung and bryophytes. There is even one record of a slime mould
growing on a living lizard! The lizard Corytophanes
cristatus is a cryptic species found in the forests of eastern
Honduras. Its 'sit and wait' foraging strategy involving periods of
immobility meant that a slime mould Physarum pusillum could
colonize its body. This lizard, which also occurs in Mexico and
Costa Rica, is the only vertebrate reported to have a plant (a
liverwort, Taxilejeunea sp.) occurring on its body.

I was under the impression that the fruiting bodies, many of which
are only millimetres high, were delicate ephemeral structures, but
some stay around for some time. When you know where to look, you can
see quite a few! For instance, in the forest near home I have found
numerous old fruiting bodies inside old stumps or in hollow logs.
One had been there long enough to have a growth of leafy liverworts
on its stem.

It is not only their sudden and sporadic appearance that is
fascinating, but also the fact that in their early stages of their
life cycle they share some characteristics with animals (i.e. they
feed and move about), while their reproductive stage is similar to
that of fungi, i.e. they produce spores.

Acellular slime moulds have two different trophic (feeding)
stages. The spores germinate into individual, soil-dwelling,
single-nucleus, sometimes flagellated amoebae. (The word amoeba
comes from the Greek amoiba: to change. It alludes to their
ever-changing shape, a result of the expansion and retraction of
temporary protrusions on their body called pseudopodia.) The amoebae
feed on bacteria and other organic matter, and then divide in two –
thus their population increases. Two compatible amoebae fuse to form
a zygote, a process that involves the fusion of the protoplasm and
the fusion of the nuclei. The diploid zygote feeds, grows and
undergoes repeated nuclear division to develop into the plasmodium
(pl. plasmodia).

Leocarpus fragilis

The plasmodia are a single cell with multiple nuclei encased in a
thin membrane. Because they can move through very small openings of
a few micrometres they are able to exploit the microhabitats within
decaying wood. There they feed on bacteria, yeasts, algae,
cyanobacteria and fungal hyphae and spores. Eventually they move to
the surface of the substrate to form fruiting bodies. This
transformation is probably triggered by exhaustion of the food
supply, and/or changes in moisture, temperature and pH. Wind
disperses the spores in most species although invertebrates
undoubtedly also play a part in this.

If conditions are unfavourable plasmodia have the ability to
transform to a hard structure (sclerotium) and revert to a
plasmodium when favourable conditions return. Similarly, amoeboid
cells have the ability to change to microcysts and back
again. Sclerotia and microcysts can remain viable for long periods;
a strategy that probably ensures their survival in arid and other
hostile habitats.

Although slime moulds are usually associated with moist conditions
and are most often observed after a bout of rainy weather they are
by no means confined to wet habitats. During an expedition to the
northern Simpson Desert in 2007 substrates were collected from the
Hay River region and taken back to incubate in the lab. Thirty-five
species were documented including nine species not previously
recorded in Australia. 41% of the species found during the
expedition, including one that is considered rare, are also found in
the desert of Western Kazakhstan, once again reflecting their
cosmopolitan distribution.

Most slime moulds are not slimy, nor do they look like mould;
rather, many are exquisitely shaped and quite beautiful. My search
for slime moulds continues and while looking for these tiny
organisms I have encountered so many other fascinating
things. Rotting wood, stumps, logs and leaf litter abound with life!
It makes you wonder about the absurdity of the notion of 'waste'
on the forest floor.

Web:
- Discover Life probably has the most extensive
collection of slime mould images on the web (linked from the main page)
- University of
Arkansas
- Introductory page
on myxomycota from University of Hawaii
- New Zealand's The
Hidden Forest has many identified moulds on their site
- The Russians have some beautiful images of slime moulds. (warning:
the file contains 33 images totalling 5.3MB.

Acknowledgements: I am extremely grateful to Paul George who has
answered many of my slime mould questions; and to Penelope
Greenslade for identifying the collembola.